1. Field of the Invention
The present invention relates generally to navigational systems, and more particularly, to a fault detection and exclusion (FDE) system for use in a navigational system utilizing multiple ranging signals to determine a position of an object.
2. Description of Related Art
Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), are typically used for navigational purposes for a variety of mobile vehicles in which relatively accurate position data is required. For instance, the GPS system is used in aircraft for guidance and navigation, in land vehicles for navigation, and in marine vehicles for navigation. A typical GPS system includes: (1) a nominal 24 satellite constellation that is positioned in six earth-centered orbital planes; (2) a ground control/monitoring network; and (3) various GPS receivers. The GPS satellites are known to use direct sequence, spread spectrum modulation for transmission of ranging signals and other navigational data. The ranging signals broadcast by the satellites are modulated with pseudo-random noise (PRN) codes that are replicated by the GPS receivers. The broadcast ranging signals generated by the GPS satellites are subject to significant errors due to anomalies in the satellite clocks, broadcast data, and atmospheric contributions, such as ionospheric and tropospheric effects. Other types of systems that provide ranging signals for navigation purposes also have similar issues.
These errors, as well as other errors in signals transmitted from GPS satellites and other ranging devices, can cause significant problems in navigation systems, such as aircraft navigation systems, which use GPS receivers and GPS satellite signals to calculate a navigation solution. For instance, an aircraft's navigation solution represents the calculated position of the aircraft in three-dimensional space at a particular time, plus heading and speed information. Navigation solution integrity, which is the guarantee to some specified high confidence level that some scalar measure of navigation solution position error (e.g., horizontal, vertical) is below a threshold called the Horizontal Alarm Limit (HAL), is essential. The function or device that ensures navigation solution integrity performs a computation of the current estimated position error and also continuously monitors a variable that is indicative of navigation solution integrity also known as the Horizontal Protection Level (HPL).
Due to potential inaccuracies in the navigational data caused by various GPS anomalies and/or other ranging sources, aircraft are typically provided with a backup navigational system that is used when the navigational data provided by the primary system, (e.g., GPS system), is not sufficiently accurate for navigational purposes. For example, many GPS receivers are provided with a self-contained Receiver Autonomous Integrity Monitoring (RAIM) system for the detection of satellite anomalies that would cause the error in the computed position to grow and possibly exceed a predetermined threshold considered safe for the current phase of flight. RAIM is one method of monitoring the integrity of a GPS navigation solution for position and time. The objective of RAIM is detection of a fault/failure condition to protect the navigation solution against the effect of an unbounded, pathological bias in any one measurement (i.e., from a GPS satellite signal) that is used as an input to the navigation solution. RAIM accomplishes this by monitoring the consistency of redundant position measurements data in an over-determined navigation position solution.
RAIM is typically implemented in software in the GPS receiver and employs an instantaneous self-consistency check. In order for RAIM to function as intended, a minimum plurality of satellite or other ranging signals are required. Where such a minimum plurality of ranging signals is not available, the RAIM internal consistency check is not available; therefore, horizontal position integrity information is not available. In addition, RAIM may also generate error values based upon the consistency check, which are then compared to predetermined error limits. Accordingly, should an error value exceed the corresponding allowable, error limit, a RAIM alarm may be generated to indicate the failure of the consistency check. This alarm is a warning to the user that although horizontal position data may be available, it may be erroneous. In such instances, where RAIM is not available or a RAIM alarm is generated, the integrity of the navigation solution is questionable.
When the RAIM function, also known as the Fault Detection (FD) function, is available, RAIM then can offer two levels of integrity capability, which differ in terms of action each undertakes following the determination that the RAIM function is available. As is well known to those in the art, the RAIM function is determined to be available if the computed protection level is less than the alarm limit against which protection is sought. The first of these two RAIM function capabilities, assuming the RAIM function is available, indicates there are no faulty measurements and therefore the position error is expected to be rather small, or alerts the user that a faulty measurement has been detected (to a specified probability) and the reported GPS position solution accuracy may not be within a pre-specified tolerance. If the RAIM function detects a faulty measurement, the RAIM function is unable to determine which of the measurement(s) (one or more) is faulty. The second of these RAIM function integrity capabilities, called Fault Detection and Exclusion (FDE), attempts to continue GPS navigation with integrity following fault detection. FDE attempts to identify the faulty measurement and exclude it from use in the navigation solution. If the faulty measurement cannot be identified, RAIM simply provides the FD level of integrity, i.e., issues an alert/alarm to the user.
Prior art FDE systems typically identify and exclude satellites by: (1) determining whether fault detection is available; (2) if fault detection is available, determining whether a fault has occurred—this is done by determining whether a “residual error” (typically calculated using the statistically-based least squares residual (LSR) method) is greater than a predetermined threshold value; (3) if the residual error is greater than a predetermined threshold value, identifying a failed satellite that is contributing to the residual error; (4) once the failed satellite is identified, excluding this satellite's measurement from the position computation; and (5) if the failed satellite can not be identified within a given amount of time, alerting the user that the reported estimated position error is greater than the allowable position error for an allowed amount of time or that the GPS navigational system is no longer operating within the required integrity. It is also possible to exclude enough satellite(s) to render the detection function unavailable.
Prior art systems are known to delay the FD function until the position error has grown to be close to, yet below, the allowable position error for the given operation. This is done to improve the probability of correctly detecting a fault because the faulty satellite's influence on the position has grown as the position error has increased. Attempts to isolate the faulty signal too early can result in an incorrect identification, because the errors have not become significantly large enough to accurately detect which signals are in error. Thus, although there is a need to detect a fault as soon as possible in the interest of operational safety, attempting isolation of the faulty signals prematurely leads to incorrect determinations of faulty signals.